1. Field of the Invention
This invention relates to a non-electrification polymeric composite material capable of being suitably used, for example, to hold a semiconductor substrate.
2. Description of the Prior Art
In producing a semiconductor device, there are performed a series of steps, such as etching a semiconductor wafer, washing the wafer, and the like, with the semiconductor wafer being held by, for example, an etching holder and carrier holder. Heretofore, as the material comprising such a holder for holding the semi-conductor wafer, there has been used a chemically resistant and heat-resistant fluoro resin such as polytetrafluoroethylene (referred to as PTFE) and a copolymer of perfluoro(alkyl vinly ether) and tetrafluoroethylene (PFA), or the like. Each of such polytetrafluoroethylene (PTFE) and copolymer of perfluoro(alkyl vinyl ether) and tetrafluoroethylene (PFA) is excellent in electric insulation and has an electric resistivity as high as 10.sup.18 .about.10.sup.19 .OMEGA..cm at room temperature, and hence is readily electrified due to friction.
Accordingly, for example, in the case where a carrier holder comprising such a fluoro resin material is dried by utilization of centrifugal force and rotated at a high speed, the carrier holder is charged with static electricity due to friction with air molecules. Consequently, the adjacent dust, dirt, or the like is attracted to the thus electrified holder and sticks to the surface of the semiconductor wafer leading to a decreased yield of semiconductor chips.
A technique to overcome such a problem is described in Japanese Laid-Open Patent Application No. 58-207651. This prior art discloses that mixing an electric conductor such as carbonaceous fiber, carbon black, or the like into the copolymer of perfluoro(alkyl vinyl ether) and tetrafluoroethylene (PFA) produces a composite material which does not lose its chemical and heat resistance in respect to the fluoro resin contained therein and at the same time exhibits a non-electrification characteristic per se.
This prior art, however, has the following problems. A copolymer of perfluoro(alkyl vinyl ether) and tetrafluoroethylene (PFA) generally used currently includes a tetrafluoroethylene copolymer containing 3.0.about.3.8% by weight perfluoro(alkyl vinyl ether). The melt index of such copolymer of perfluoro(alkyl vinyl ether) and tetrafluoroethylene (PFA) is 1.5-18 g/10 min. When an electric conductor such as carbon black is mixed in the copolymer of perfluoro(alkyl vinyl ether) and tetrafluoroethylene (PFA) to obtain this non-electrification characteristic, the mechanical strength of the copolymer is lowered and the melt viscosity is increased. For example, when 8% by weight acetylene black based on the copolymer of perfluoro(alkyl vinyl ether) and tetrafluoroethylene (PFA), the electrically conductive carbon being excellent in non-staining properties and containing very little impurities, is added to the copolymer of perfluoro(alkyl vinyl ether) and tetrafluoroethylene (PFA), the melt index becomes 40-50% of the melt index of the copolymer of perfluoro(alkyl vinyl ether) and tetrafluoroethylene (PFA) before the addition of the acetylene black. For injection molding, generally, the melt index should be about 10 g/10 min or more. For injection molding, accordingly, the melt index of the copolymer of perfluoro(alkyl vinyl ether) and tetrafluoroethylene (PFA) before the addition of the acetylene black must be 20 g/10 min or more.
The copolymer of perfluoro(alkyl vinyl ether) and tetrafluoroethylene (PFA) used so far does not satisfy the above melt index with respect to mechanical properties. Therefore, it is inadequate to use the conventional copolymer of perfluoro(alkyl vinyl ether) and tetrafluoroethylene (PFA) and add acetylene black as the material for a holding wafer in the semiconductor device, or the like, since it is poor in molding properties.
There is a corelationship between the similar materials, that is, the smaller the molecular weight, the larger the melt index. For this reason, it is considered to use a copolymer of perfluoro(alkyl vinyl ether) and tetrafluoroethylene (PFA) comprising constituents relatively small in molecular weight and having a melt index of 20 g/10 min or more. In this case, however, though the large melt index may elevate the molding properties, the mechanical properties of the non-electrification polymeric composite material after the addition of the electrically-conductive carbon remarkably deteriorates the mechanical properties of the non-electrification polymeric composite material causing, for example, cracks after molding.